Abstract Chemical remagnetization is a very common phenomenon in sedimentary rocks and developing a greater understanding of the mechanisms has several benefits. Acquisition of a secondary magnetization is usually tangible evidence of a diagenetic event that can be dated by isolation of the chemical remanent magnetization and comparison of the pole position to the apparent polar wander path. This can be important because diagenetic investigations are frequently limited by the difficulty in constraining the time frames in which most past events have occurred. Remagnetization can commonly obscure a primary magnetization; developing a better understanding of remagnetization could improve our ability to uncover primary magnetizations. Many chemical remagnetization mechanisms have been proposed, including those associated with chemical alteration by a number of different fluids (orogenic, basinal and hydrocarbons), burial diagenetic processes (clay diagenesis and maturation of organic matter) or other processes. This paper summarizes our current knowledge of these chemical remagnetization mechanisms, with a focus on examples where there is a connection with chemical alteration.

Abstract Authigenic formation of fine-grained magnetite is responsible for widespread chemical remagnetization of many carbonate rocks. Authigenic magnetite grains, dominantly in the superparamagnetic and stable single-domain size range, also give rise to distinctive rock-magnetic properties, now commonly used as a ‘fingerprint’ of remagnetization. We re-examine the basis of this association in terms of magnetic mineralogy and particle-size distribution in remagnetized carbonates having these characteristic rock-magnetic properties, including ‘wasp-waisted’ hysteresis loops, high ratios of anhysteretic remanence to saturation remanence and frequency-dependent susceptibility. New measurements on samples from the Helderberg Group allow us to quantify the proportions of superparamagnetic, stable single-domain and larger grains, and to evaluate the mineralogical composition of the remanence carriers. The dominant magnetic phase is magnetite-like, with sufficient impurity to completely suppress the Verwey transition. Particle sizes are extremely fine: approximately 75% of the total magnetite content is superparamagnetic at room temperature and almost all of the rest is stable single-domain. Although it has been proposed that the single-domain magnetite in these remagnetized carbonates lacks shape anisotropy (and is therefore controlled by cubic magnetocrystalline anisotropy), we have found strong experimental evidence that cubic anisotropy is not an important underlying factor in the rock-magnetic signature of chemical remagnetization.

Abstract Remagnetization of a palaeomagnetic signal is difficult to recognize independently of directional information. The situation becomes more complex when remagnetized rocks pass palaeomagnetic field tests, for example when the remagnetization of a rock sequence has occurred before folding. It is evident that palaeogeographic reconstructions are seriously flawed when actually remagnetized rocks are not identified as such. Here we discuss the merits and pitfalls of so-called end-member modelling of acquisition curves of the isothermal remanent magnetization (IRM) to recognize remagnetized strata. The technique requires no a priori information about the IRM acquisition curves. The algorithm unmixes a set of IRM acquisition curves into a number of invariant curves termed end members and calculates the mixing proportions of the end members for each sample. Since primary natural remanent magnetization (NRM) and remagnetized NRM are acquired by different processes, their signatures can be recognized from subtle differences in the magnetic properties. We illustrate the potential of the approach by three case studies, one from Spain and two from Turkey, in which the magnetic properties of remagnetized and non-remagnetized rocks are evaluated.

Abstract With increasing interest in the middle Devonian Marcellus Shale as a gas play in the Appalachians, a study of the rock magnetic characteristics and remanence components was undertaken. Samples were collected from outcrops of the Union Springs Formation and the underlying Onondaga Formation in a syncline immediately east of the Broadtop synclinorium in the Valley and Ridge province in Pennsylvania. The rocks contain an intermediate-temperature (IT) component with south-southeasterly declinations and shallow up inclinations that was removed by 310–350 °C, interpreted as a chemical remanent magnetization (CRM) that resides in pyrrhotite. At higher temperatures (350–480 °C) a component with more southerly declinations and shallow down inclinations, interpreted as a CRM in magnetite, is removed. Low-temperature treatments resulted in more stable decay during thermal demagnetization which allowed the IT component to be more easily identified. Cumulative log-Gaussian analysis of an isothermal remanent magnetization (IRM) and triaxial thermal decay of the IRM indicate the presence of two dominant minerals: pyrrhotite and magnetite. Low-temperature saturation IRM experiments show the 32 K Besnus transition and saturation of magnetization between 200 and 250 K, indicative of pyrrhotite. Some specimens showed the Verwey transition at 120 K, indicating magnetite.

Abstract Chemical remagnetization is a very common phenomenon in sedimentary rocks and developing a greater understanding of the mechanisms has several benefits. Acquisition of a secondary magnetization is usually tangible evidence of a diagenetic event that can be dated by isolation of the chemical remanent magnetization and comparison of the pole position to the apparent polar wander path. This can be important because diagenetic investigations are frequently limited by the difficulty in constraining the time frames in which most past events have occurred. Remagnetization can commonly obscure a primary magnetization; developing a better understanding of remagnetization could improve our ability to uncover primary magnetizations. Many chemical remagnetization mechanisms have been proposed, including those associated with chemical alteration by a number of different fluids (orogenic, basinal and hydrocarbons), burial diagenetic processes (clay diagenesis and maturation of organic matter) or other processes. This paper summarizes our current knowledge of these chemical remagnetization mechanisms, with a focus on examples where there is a connection with chemical alteration.

Abstract Remagnetizations have been recognized ever since magnetizations in rocks were demonstrably shown to have been acquired at a much later time than the formation or deposition of the rocks themselves. There was mention of remagnetizations as early as the 1950s, and in the 1960s the concept was frequently hypothesized as an explanation for repetitions and loops in apparent polar wander paths. In this paper, remagnetization features and processes are organized by magnetic carrier: hematite, magnetite, Fe-sulphides and goethite. Selected case histories are presented which are chosen in order to reveal important diagnostics, although many origins of remagnetizations are still obscure or incompletely known.

Abstract The South American record of remagnetizations is linked to specific events of its tectonic history stretching back to Precambrian times. At the Ediacaran–Cambrian time interval (570–500 Ma), the final stages of the western Gondwana assemblage led to remagnetization of Neoproterozoic carbonates within the São Francisco–Congo Craton and at the border of the Amazon Craton, along the Araguaia–Paraguay–Pampean Belt. From the late Permian to early Triassic, the San Rafaelic orogeny and the emplacement of the Choiyoi magmatic province was responsible for widespread remagnetizations in Argentina and Uruguay. Cretaceous remagnetization has also been documented in Brazil and interpreted to result from magmatism and fault reactivations linked to the opening of the South Atlantic Ocean. We present a review of these widespread remagnetization events principally based on palaeomagnetic data and, when available, on rock magnetic and radiogenic isotope age data. This study gives an overview of the geographical distribution of the remagnetization events in South America, and provides important clues to better understand the geodynamic evolution of the South American plate at these times. In addition, magnetic mineralogy data for the different case studies presented here constrain the physical–chemical mechanisms that led to partial or total resetting of magnetic remanences in sedimentary rocks.

Abstract A palaeomagnetic study of four oriented cores was conducted to better understand the timing of diagenetic events in the Mississippian Barnett Shale, a primary source rock and the unconventional gas reservoir in the Fort Worth Basin, Texas. Thermal demagnetization removes a present-field modern viscous remanent magnetization (VRM) as well as a chemical remanent magnetization (CRM) that has shallow inclinations and streaked south–SE-directed declinations. The VRM was used to orient the CRM data for one well and it produced a similar streak of directions. The streaking of directions could represent a mixing trend between two or more CRMs. Specimens from bedding-parallel and NE subvertical mineralized fractures contain a CRM that is interpreted to be of Pennsylvanian age and to have formed in response to burial diagenetic processes. NE- and NW-oriented vertical fractures are common to rocks that contain late Permian to Triassic CRMs. Sr and sulphur isotope results from vein minerals around NE fractures suggest the CRM could be related to fluids sourced from the Ouachita front. The SE directions in the streak could be explained if the northern part of the basin experienced a component of anticlockwise rotation of up to 20° in the Pennsylvanian.

Abstract Palaeomagnetic and rock magnetic data collected from the Upper Ordovician Red River, Silurian Interlake and Devonian Winnipegosis, Souris River and Birdbear carbonates in one well from southwestern Manitoba (Canada) reveal a complex magnetization history for the north-eastern Williston Basin. Rock magnetic analysis (thermal demagnetization, anhysteretic remanent magnetization (ARM)/saturation isothermal remanent magnetization (SIRM), S-ratios, partial ARM (pARM), SIRM crossover curves and points and coercivity) show three magnetic carriers for different magnetizations seen in this well. An Early–Mid-Jurassic remagnetization observed in the lower Red River and Souris River formations is carried by single-domain to pseudo-single-domain (SD–PSD) magnetite and was probably produced by basement fluids circulating along fractures and faults created by the Hartney impact/volcanic structure and/or tectonic movements along the Superior Boundary Zone. In the Winnipegosis Formation a possible primary depositional or early depositional magnetization (Devonian age) is carried by PSD pyrrhotite. In the Birdbear Formation two different magnetizations of uncertain age are present: one carried in hematite in the upper strata, possibly originating from the younger Amaranth Formation, and magnetite dominates in the lower strata. The upper Red River and Interlake formations contain both magnetite and pyrrhotite; however, the weaker palaeomagnetic data reveal little in terms of a magnetization age.

Abstract An integrated palaeomagnetic, geochemical and petrographic study was conducted on two folds in the Front Range of the Southern Canadian Cordillera in order to better understand the timing and origin of chemical remanent magnetizations (CRMs) relative to orogenesis. The folds are contained within Mississippian carbonates (330–335 Ma) which contain a pervasive pre-tilting to early syn-tilting Early Cretaceous, high-temperature CRM residing in magnetite. An intermediate-temperature CRM is a late syn-tilting to post-tilting, possibly Tertiary remagnetization, residing in pyrrhotite. A fluid conduit test (FCT) conducted on bedding-parallel veins shows that they are associated with the magnetite CRM, whereas late-stage tensile veins show a relationship to a pyrrhotite CRM. Elevated 87 Sr / 86 Sr data indicate alteration by fluids with a radiogenic signature; along with the FCT results, these data are consistent with the interpretation that the magnetite CRM formed as a result of hydrocarbons and/or evolved basinal fluids that migrated ahead of the deformation front. Based on the presence of sulphur-enriched bitumen, barite and sphalerite, common by-products of thermal sulphate reduction (TSR), the pyrrhotite CRM is interpreted to be the result of late-stage TSR caused by warm basement fluids which moved along faults and fractures.

Abstract The Devonian Alamo Breccia is a thick (&lt;30–130 m) unit, interpreted as a bolide impact deposit, which is bracketed by marine carbonates. Samples were collected within the breccia and above/below the breccia for a contact test to determine if the breccia acted as a conduit for fluids that could have caused the widespread chemical remanent magnetizations (CRMs) present in Palaeozoic Era rocks in Nevada. The carbonates above, below and in the breccia contain a Cretaceous Period syn-tilting CRM that resides in pyrrhotite and a pre-tilting late Palaeozoic Era CRM that resides in magnetite. The contact test is negative. Despite these results, diagenetic alteration by externally derived fluids is interpreted as the most likely mechanism of remagnetization. This hypothesis is supported by 87 Sr/ 86 Sr values in the breccia and surrounding rocks that suggest alteration by fluids with a radiogenic signature. The fluids were not localized in the breccia but are interpreted to have moved pervasively through the rocks. The results differ from some other studies that found that fluids caused localized CRMs around fluid conduits.

Abstract This paper reviews results on the nature and thermo-tectonic interpretation of widespread pyrrhotite remagnetizations in the Himalaya. Throughout the last two decades a large dataset has been acquired, in particular from low-grade metamorphic rocks of the Tethyan Sedimentary Series (TSS). The nature of this magnetization is a thermoremanence when the peak metamorphic temperature T max exceeded the Curie temperature ( T c c. 325 °C) of pyrrhotite; in this case the remanence age can be related to last metamorphic cooling. For T max &lt; T c , the remanence can be of chemical, thermochemical or thermoremanent origin. Cooling ages show a systematic trend of c. 50–20 Ma from the western to the eastern Himalaya. The pyrrhotite remagnetizations post-date main Himalayan folding and record late orogenic long-wavelength rotations and tiltings around vertical and horizontal axes. Remanence directions in the western Himalaya are well matching with large-scale deformations of rotational shortening and oroclinal bending, while in the central and eastern Himalaya they are predominantly controlled by meso-scale effects due to crustal doming. Stable pyrrhotite remanences are especially typical for low-grade marly limestones in the TSS, but were also found in medium-grade rocks of the Lesser Himalaya, highly metamorphic rocks of the Higher Himalayan Crystalline and diorite dykes intruding into the TSS.

Abstract We investigate the broad lines of magnetic mineral formation for non-metamorphic claystones. More particularly, we focus on the formation of magnetite from c . 20 day-experiments aiming to reproduce burial conditions. It is shown that the sole action of temperature from 50 °C to 250 °C leads to the formation of magnetite. The neoformed magnetites carry a chemical remanent magnetization which equates at least the natural remanent magnetization. We propose a schematic of burial with three magnetic windows where a chemical remanent magnetization superimposed the natural remanent magnetization. These are the greigite window (subsurface), the magnetite window (depth &gt; 2 km) and the pyrrhotite window (depth &gt; 6 km). The formation of magnetic minerals has profound consequences for the magnetostratigraphy record. We propose a conceptual model that shows that the continuous production of magnetite during burial may result in magnetozones that have no relation to the age of the sediment.

Abstract This study combines magnetic experimentation and geochemical analysis on oil sands from Osmington Mills and Mupe Bay, Wessex Basin, UK to investigate the possibility of a relationship between hydrocarbons and magnetic mineralogy. Removal of hydrocarbons by chemical extraction was conducted to allow comparison of (1) oil sands and (2) cleaned sands. Detailed magnetic analysis including low-temperature and high-temperature experimentation revealed that all but one sample was dominated by siderite, identified by the Néel transition at 37–38 K as well as containing large grains of multidomain magnetite (Verwey transition 110 K) and hematite (Morin transition 250 K). Scanning electron microscopy and energy dispersive x-ray analysis confirmed the presence of iron oxides, in particular framboids 500 nm–45 µm in diameter, probably magnetite. Hysteresis parameters showed distinct grouping of oil sands compared to their clean counterparts and a negative linear regression in log space was observed ( R 2 =0.7) between the percentage of extractable organic matter and magnetic susceptibility. These results suggest a relationship exists between magnetic minerals and the alteration of oil due to biodegradation, which is not yet fully understood. Possible mechanisms are suggested to be due to anaerobic bacteria or the transportation of the oil as it migrates through the host rock.

Abstract We have carried out rock magnetic characterizations of different lithofacies along the stratigraphic well Saltarín 1A, in order to learn about the various diagenetic events that could have affected the Miocene sequence of the Llanos foreland basin (Colombia). Thermomagnetic and low-temperature susceptibility measurements performed on some selected samples were complemented with analyses of Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) and isothermal remanent magnetization (IRM) acquisition curves. The identification of the magnetic mineral assemblages at each depth level analysed, as well as their relative concentrations, were determined from a direct signal analysis (DSA) of the IRM curves. Samples from the top of the Guayabo formation reveal the presence of hydrocarbons-related microscopic framboids of pyrite with partial replacement of magnetite. The bottom of the Guayabo formation shows hematite and goethite and appears to record a thoroughly documented Middle Miocene global regression. In samples from the León formation, pyrrhotite could have resulted from an early diagenesis that took place in a lacustrine environment via sulphate reduction. Traces of crude oil in samples from the Carbonera formation, and the additional occurrence of hematite and magnetite, suggest that a hydrocarbons-mediated late diagenesis could also have affected the lowermost levels of Saltarín 1A.

Abstract The Golfo San Jorge Basin is one of the most important hydrocarbon-producing basins in Argentina. A study of magnetic properties performed on drill cutting from an oil well drilled in this basin was carried out. The cutting samples, taken from an interval of about 400 m thickness, correspond to the upper units of the Pozo D-129 Formation, the main source rock of the basin. Magnetic susceptibility measurements were made and rock-magnetism studies were conducted. Concentration indices of the different magnetic species determined were calculated based on isothermal remanent magnetization acquisition curves. A correlation analysis among magnetic properties, hydrocarbon content and well logs (sonic, neutron, density, induction, resistivity and photoelectric factor) was performed. Several kinds of significant correlations were found: a positive correlation between susceptibility and relative hydrocarbon content; a positive correlation between magnetic properties and porosities (especially good with the neutron log porosity); and a negative correlation between the concentration indices of some magnetic species (magnetite and pyrrhotite) and resistivity. Pyrrhotite could be directly related to the presence or migration of hydrocarbons through the porous units. The qualitative correlations between magnetic data and key petrophysical parameters such as porosity, along with the association of magnetic mineralogy to hydrocarbon presence or migration, suggest the potential usefulness of these techniques for subsurface exploration.